Magneto-optical analog to digital signal converter



Feb. 18, 1964 I. F. BARDITCH MAGNETO-OPTICAL ANALOG TO DIGITAL SIGNALCONVERTER Filed May 5, 1960 2 Sheets-Sheet 1 3s 37 l8 38 I6 32 39AMPLIFIER 33 34 i- LQ as Flg. l.

3 5| I? 50 I3 l9 23 2| 27 STANDARD 25 DIRECT CURRENT 22 VOLTAGE 1 9 I'M?I 1| i 74 2G Fug. 2. 4% 4 l2' h 4 wnmzsses INVENTOR Irving F. Bordirch.

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ATTORNEY Feb. 18, 1964 F. BARDITCH 3,121,866

MAGNETO-OPTICAL ANALOG T0 DIGITAL SIGNAL CONVERTER Filed May 5, 1960 2Sheets-Sheet 2 Fig. 4.

United States Patent "ice 3,121,866 MAGNETO-0PTICAL ANALOG T0 DIGITALSIGNAL CONVERTER Irving F. Barditeh, Baltimore, Md., assignor toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvania Filed May 5, 1960, Ser. No. 27,092 19 Claims. (Cl.340347) This invention relates to improvements in analog to digitalconverters, and more particularly to an improved magneto-optical analogto digital voltage converter.

Prior art devices for converting from analog to digital type of datapresentation have been characterized by a number of disadvantages, andthe apparatus of the instant invention overcomes these disadvantages byeliminating switching, differencing, counting, or any of the otherprevious techniques which have been employed. The device is capable ofhigh speed operation and may be made in substantially monolithic form.

Accordingly, a primary object of the instant invention is to provide anew and improved analog to digital signal converter.

A further object is to provide a new and improved analog to digitalvoltage converter utilizing magneto-optical effects.

A further object is to provide a new and improved magneto-optical analogto digital voltage converter which may be made in substantiallymonolithic form.

These and other objects will become more fully apparent after a study ofthe following specification, when read in connection with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic View partially in perspective of apparatusaccording to the preferred embodiment of the invention;

FIG. 2. is a cross section of a portion of the apparatus of PEG. 1;

FIG. 3 is a diagrammatic view of a suitable source of polarized lightfor use in the invention; and,

FIG. 4 is a further view of a portion of the apparatus of PEG. 1 showingthis portion in greater detail.

Referring now to the drawings for a more detailed understanding of theinvention, in which like reference characters are used throughout todesignate like parts, there is shown at It) a member composed ofmagnetooptically active material characterized by the magnetoopticaleffect. The term magneto-optical effect is applied to the rotation ofthe direction of the electric vector of plane-polarized light byinteraction with a magnetic field while traversing a suitable medium.Member it} may be a film of yttrium or gadolinium iron garnet. Severalferrimagnetic oxides including yttrium or gadolinium iron garnet aretransparent to light in the infrared and visible-light frequencies. Thecharacteristic of yttrium iron garnet of particular interest to theinstant invention is the property of the material while subjected to anelectromagnetic field of altering the polarization of light passingthrough the material in the region where the magnetic field is active oris maintained. This characteristic of yttrium iron garnet has beendescribed in the literature, for example, reference may be had to anarticle, entitled Ferromagnetic Resonance and Non-Linear Effects inYttrium Iron Garnet, by Le Craw, Spencer, and Porter, Journal of AppliedPhysics, vol. 29, March 1958, pages 326-327. Additional articles in thesame issue of the same periodical are entitled Trans-parentFerromagnetic Light Modulator Using Yttrium Iron Garnet, by Porter,Spencer and Le Craw, pages 495 496, and Optical Properties of SeveralFerrimagnetic Garnets, by Dillon, pages 539-541.

More particularly, magneto-optically active material 3,121,866 PatentedFeb. 18, 1964 includes material characterized by either one or both ofthe effects of :faraday rotation or magnetic birefringence, the formerof which will be discussed in more detail hereinafter.

As a result of the faraday rotation effect, a plane-polarized wave willundergo rotation on passing through a magnetic medium that is magnetizedin the direction of propagation of the wave, i.e. the magnetic fluxlines are parallel to the Poynting vector of. the light.

The magnetic structure of the actual crystals of the yttrium iron garnetor gadolinium iron garnet material in its natural state may be broken upinto magnetically discrete or saturated regions called domains. Byproper processing or treatment of the material, the domains of thegarnet crystal may be made to lie so that their mag- .etic moments areessentially perpendicular to the surface of the crystal, although theyare not pointing in the same direction but generally in oppositedirections, that is, the lines of force of the fields of adjacentdomains may be parallel but the polarities may be opposite.Plane-polarized light normal to such a crystal surface will then berotated in different portions of the crystal by amounts and in angulardirections depending on whether the domain magnetic moment is parallelor antiparallel (polarity reversed) to the incident beam; some of thelight will be rotated clockwise while some of the light will be rotatedcounterclockwise.

If a light analyzer, such for example as one known in the art as a Nicolprism analyzer, is set at degrees to a plane-polarized li ht wave, nolight will emerge from the analyzer; in this orientation, the analyzeris said to be set at extinction. If a garnet crystal with domainscreating magnetic moments normal to the surface is inserted between theplane-polarized light wave source and the analyzer, as previously statedsome of the domains will rotate the light clockwise and some will rotateit counterclockwise. The light emerging from the crystal is now rotatedin various portions of the beam with respect to the incident plane wave.The result is that some of these emerging parts of the beam are noworiented at such an angle that the Nicol prism is not set to extinctionfor them. Therefore, light is transmitted through the crystal and theNicol prism. By rotating the analyzer with respect to the crystal,various of the domains may be set at essentially extinction, whileothers will not be. Accordingly, some light passes through the analyzer.

The Nicol prism or any other material such as is known by the namePolaroid which can polarize light may be used.

The domain magnetization direction of being parallel or antiparallel tothe Poynting vector changes readily in the presence of a field producedby small magnets; a small external magnetic field may cause the magneticfields or moments of all the domains to lie antiparallel to thedirection of the Poynting vector of the light. Under these conditions,all the plane-polarized light passing through the garnet material can beblocked by an analyzer set at extinction for this particular condition.Removal of the field will allow the light to be transmitted again. Inother words, the application of a small external magnetic field to thegarnet material can change the light transmitted through the analyzerfrom on to off.

By cutting a yttrium-iron-garnet crystal (formula: 5Fe O 3Y O to yield athin slice having given crys tallographic planes parallel to thesurface, the magnetic moments of the domains may be arranged so that inzero applied field they are directed as desired.

The member 10 has disposed thereon or therein a plurality of preferablyparallel transparent leads or condoctors of small diameter designated inFIG. 1 by the reference numerals 11, 12, 13 and 14. Whereas for ease ofillustration only four leads are shown in FIG. 1, it will be readilyunderstood that any desired number may be employed depending upon thedesired capacity of the analog to digital converter. The transparentleads 11, 12, 13 and 14 may be thin electrically conductive films of tindioxide deposited and baked on garnet film 10.

There is shown at 18 an amplifier to which the analog voltage to bedigitized is fed by any convenient means as by lead 16. The amplifier 18develops an output voltage between ground 17 and each of leads 11-15which is proportional either to variations in the analog quantity orsignal, or which is a function of the value of the analog quantit as maybe desired. It will be observed from FIG. 1 that one end of each of theleads 11, 12, 13, 14 and 15 is connected to the aforementioned amplifier18 output. It will be understood that the lead means 15 symbolicallyrepresents connections to any desired number of additional transparentleads in or on the member as may be desired. It is to be further notedthat the amplifier 18 is of such design that each of the leads 11, 12,13, is fed so that none interacts with any of its neighbors and each ineifect is fed independently. Suitable amplifiers of this type have beenbuilt consisting of a level of voltage amplifier feeding a number ofparallel emitter follower transistor stages each feeding its separateline 11, 12 etc.

It will be noted from FIG. 1 that the other end of each of leads 11, 12,13 and 14 is connected to spaced points 19, 2t 21 and 22 on a resistancevoltage dividing network including the resistors 28, 23, 24 and 25 andwhich is energized by way of resistor 9 and lead 26 by direct currentpotential from a standard voltage source 27 which develops a desiredstandard potential between lead 26 and ground 17. Again, these are sofed from suitable isolation sources, not shown, so as to make anycurrents drawn during operation unable to disturb the voltagedistribution at any point elsewhere in the circuit. Terminal 19, whichfor purposes of illustration only is chosen as representing the lowestvoltage to which the analog to digital converter is responsive, isconnected by way of the aforementioned resistor 28 to ground 17.

Disposed in front of the member 10 (to the left as viewed in FIG. 1) isa suitable source of polarized light, the light being of a frequency orwavelength to which film 10 is transparent, which light source mayinclude a light source 50, a polarizing element 51, and a collimatingelement '52. Disposed back of and adjacent to member 119 is an analyzeror filter member 30 for polarized light, which may be a light-polarizingfilm, and which is of the correct type to pass light waves having apredetermined polarization corresponding to the polarization of some ofthat light emanating from the source 5051 which passes through garnetfilm or member 10 in a region or area where there is substantially noexternal magnetic field. More specifically, the filter for polarizedlight 30 is set at extinction for light having a plane of polarizationat a predetermined angle to the plane of polarization of light leavingsource 50-51. Disposed adjacent to and back of member 311 are anydesired number of photosensitive readout devices, five devices beingshown in FIG. 1 and designated 31, 32, 33, 34 and 35. Lead connections36, 37, 33, 39 and 40 are made respectively to the upper ends of thephotosensitive devices, and it will be understood that lead connections,not shown in FIG. 1, may be made to the lower ends of the devices.

In the operation of the apparatus of FIG. 1, it will be noted that theright-hand end of each of the leads 11, 12, 13 and 14, as viewed in FIG.1, is at a slightly different potential with respect to the other leads,as a result of the direct current source 27. For the purposes of thisdescription and to facilitate an understanding of the invention, theamplifier 18 may be presumed .9 have an output impedance which isresistive in nature. Assume now by way of description that the amplifier18 develops on leads 11-14 ajdirect current voltage as a result of someinput thereto, and that this direct current voltage on leads 11-14corresponds substantially to the voltage at one of the points 19, 20,21and 22. For purposes of illustration, let it be assumed that the voltage on the left-hand end of lead 12 is the same as the voltage atterminal or point 241. No current will flow in lead 12 whereas, becauseof a difference of potential between the two ends of each of the otherleads 11, 13 and 14 a current will flow either from a left-to-rightdirection or from a right-to-left direction. The magnetic fields set upby these currents cause the magnetic moments of the domains to assumesubstantially uniform directions and polarities, so that all the lightpassing through member 10 in the areas adjacent the current carryingleads is rotated in the same angular direction to form a plane ofpolarization at an angle for which filter element 36 is set atextinction. Accordingly, no light passes through member 30 in the areasthereof back of leads 11, 13 and 14. However, at the same time, nocurrent is flowing in the lead 12 because the potentials at the two endsthereof are substantially equal in amplitude and of like polarity, andno magnetic field is set up in the area or region of member 10 adjacentlead 12. Accordingly, light from source 5li51 arriving at member 10 inthe area adjacent lead 12 does not have the polarization of a portionthereof altered in an adverse manner, and this portion passes throughthe analyzer element 30 and impinges upon all of the photosensitivereadout devices 31, 32, 33, 34 and 35. Accordingly, light passes throughmembers 10 and 30 only in a rectangular area, zone or strip whichcorresponds to a particular voltage, and the member 10 may for purposesof analysis be thought of as constituting a number of parallel strips,each of which passes some light without altering the polarizationthereof in a manner to be extinguished at 30 when a voltage of aparticular amplitude is applied to leads 11-14 from amplifier 18.

Particular reference is made now to FIG. 4 which shows thephotosensitive readout devices 31, 32, 33, 34 and 35 in greater detail.The five photosensitive devices 31, 32, 33, 34 and 35 provide for abinary notation or binary counting including five digits. It will bereadily understood that five devices are used for ease of illustrationand that any number of desired photosensitive devices may be utilizeddepending upon how high it is desired to count. Each of the rectangularzones, lying at intervals in a longitudinal direction, and extendingtransversely across the entire five devices, has five portions which areselectively masked in accordance with a binary coding pattern, so thatthe photosensitive devices are selectively energized in accordance witha binary code pattern. For example, in accordance with conventionalbinary code where five digits are employed, the number 1 would berepresented by 00001. Accordingly, the rectangular zone 41 adjacent thelead 11 has the first four device portions (reading from left to right)masked in that zone by any suitable means, and the portion of the lastphotosensitive device 35 is unmasked. Light passing through member 16 inthe area adjacent lead 11 and thence through member 30 and impingingupon only the zone 41 would therefore cause only the fifth or lastphotosensitive device 35 to produce an output signal on lead 40. Therectangular zone encompassing portions of the five devices lyingadjacent the aforementioned lead 12 is designated 42, and in accordancewith conventional binary coding, the number 2 is represented in binarynotation by 00010. Accordingly, the portions in the rectangular zone 42of photosensitive devices 31, 32, 33 and 35 are masked in the areasthereof adjacent lead 12 while the adjacent portion of photo sensitivedevice 34 is unmasked, and when no potential difference exists betweenthe ends. of lead 12, light passes s,121,see

through member and analyzer member and impinges upon only the unmaskedportion of device 34 adjacent the lead 12, and only the output lead 39of the five output leads has a signal produced thereon.

Particular reference is made now to FIG. 2 which is a cross section of aportion of the apparatus of FIG. 1 as it may be constructed inmonolithic form. The leads 11' and 12' may be deposited leads of tindioxide deposited on a film 10' of yttrium iron garnet and bakedthereon. Adjacent the film 10' is a transparent backing plate 71 of anyconvenient or suitable material, which may be omitted if desired, andadjacent backing plate 71 is the analyzer element 30 which may asaforementioned be light-polarizing film of the correct type, whereasadjacent the back surface of member 30 is a partially filled space 72,which may be used if desired and may provide room for the maskingmaterial on the aforementioned photosensitive elements or devices, orthe masking material may be deposited or otherwise applied to theelements so that no space 72 is required. Photosensitive element 31 issupported on the back thereof by any suitable mounting means 73. Ifdesired, the readout devices may be selenium film or cadmium sulfideevaporated on the back of polaroid film 3t and portions of the analyzer30 may be masked before evaporation of the photosensitive readout stripsso that the same final result is achieved, that is, the provision ofsensitive and insensitive portions.

Particular reference is made now to FIG. 3. A suitable source ofpalarized light may comprise a light source 74 disposed in the reflectormeans 75 and having a polarizing lens 76 adjacent thereto.

The number of degrees rotation of the polarization of the incident lighton member 10, while not too sensitive to changes in the strengths of themagnetic fields set up at the leads, is, for any given magneto-opticallyactive material, a function of the thickness of the material and theWavelength of the light. The invention contemplates suitable choice ofmaterial, dimensions including thickness, and wavelength, to provide thedesired manner of operation in accordance with the characteristics ofthe analyzer 30.

There has been provided then apparatus well suited to accomplish theaforedescribed objects of the invention. A variation in the voltage onleads 11-14 occurs as a result of changes in the input to amplifier 18on lead means 16. Assuming one of the leads 11, 12, 13 or 14 has nocurrent flow therethrough as a result of the voltage developed on thelead, a change in the signal input on lead means 16 will cause adifferent one of the leads l1, 12, 13 and 14 to have substantially nocurrent therethrough because of the equal potential existing at the twoends thereof, and as a result, another lead, for example, lead 13,having no current flow therethrough will cause a shift in thepolarization of only a portion of the light arriving at the areaadjacent thereto from source Sit-51 in a manner tobe excluded byanalyzer 30, the remaining portion passing through analyzer 30. Thislast-named light portion is passed to the adjacent portions of thephotosensitive readout devices 31, 32, 33, 34 and 35, causing the outputleads 36, 3'7, 38, 39 and 40, respectively, to have signals or nosignals thereon in accordance with whether the sections or portions ofthe devices are masked in the particular areas where light falls thereonin accordance with the pattern of FIG. 4, and a direct readout on eachof the aforementioned five output leads corresponding to whether thatparticular binary digit is O or 1 is obtained. No moving parts arerequired. The apparatus may be made sensitive to small changes in thepotential or signal on lead 16, and by using any desired number ofphotosensitive readout devices with the corresponding required number ofleads on member 10, the apparatus may be made to have a capacity of anydesired binary count.

The invention includes the use of additional resist ance networks andisolation devices desired to insure uniform potential steps betweenadjacent leads on memher it), and to provide for operation in the mannerintended.

The invention includes the use on? light guiding and shielding meansWhere desired to insure that light passing through any given portion ofmember 10 or member 30 arrives at only its intended destination.

Whereas the invention has been shown and described with reference toyttrium and gadoliniumdron garnet crystals, it will be understood thatother suitable magneto optically active materials may be employed.

It will be understood that as an obvious adaptation the photosensitivedevices could be coded to give a decimal readout as well as a binaryreadout; for example 19 adjacent readout devices could provide fordecimal counting to 99, a suitable number of leads being provided onmember 10.

In final summary, in the absence of a magnetic field, some domains ofthe yttrium iron garnet or other active material rotate a portion of thelight passing therethrough in one angular direction while other domainsrotate the remaining portion of the light in the opposite angulardirection; one analyzer can be set to block only one portion, and theother portion passes to the photosensitive readout devices. A smallexternal magnetic field causes the magnetic moments of all the domainsto assume uniform directions and polarities; all the light passingthrough the active material is rotated in the same angular direction andis excluded by the analyzer. l

Whereas the invention has been shown and described with respect to anembodiment thereof which gives satisfactory results, it should beunderstood that changes may be made and equivalents substituted Withoutdeparting from the spirit and scope of the invention.

1 claim as my invention: 1

1. In analog to digital signal converter apparatus, in combination, asource of light having a predetermined plane of polarization, aplurality of light sensitive ele men-ts disposed in predeterminedpositions with respect to each other and with respect to said source outlight, each of said light sensitive elements having sensitive andinsensitive portions and generating an electrical output signal whilelight energy impinges only on a sensitive portion thereof, the sensitiveand insensitive portions of all said plurality of light sensitiveelements being arranged in a predetermined pattern whereby lightreaching certain selected sensitive portions provides output signalsfrom the light sensitive elements which are digitally coded tocorrespond to a quantity in accordance with the arrangement of theselected sensitive portions with respect to each other and lightreaching certain other selected sensitive portions provides other ouputsignals from the light sensitive elements which are digitally coded tocorrespond to another quantity in accordance with the arrangement of theother selected sensitive portions with respect to each other, lightfiltering means interposed between the source of light and the pluralityof light sensitive elements and excluding only light having a plane ofpolarization at a predetermined angle with respect to said predeterminedplane of polarization, and analog signal means including magnetooptically active means interposed between the source of light and saidlight filtering means for selectively controlling the plane ofpolariza-tion of light reaching the light filtering means in selectedareas thereof selected in accordance with the value of the analog signalto thereby variably select groups of sensitive portions to beilluminated in accordance with variations in the value of the analogsignal.

2. In analog to digital signal converter apparatus, in combination, aplurality of lead means, means connected to all of said plurality oflead means for causing current flows in all but a selected one of saidlead means selectively in accordance with the value of the analog signalto be digitized, magneto-optically active means disposed adjacent all ofsaid lead means, those of said lead means having current flowstherethrough creating magnetic fields in the adjacent regions of themagneto-optically active means, polarized light filter means disposed onone side thereof in predetermined position with respect to saidmagneto-critically active means, photo-sensitive means disposed on theother side of said polarized light filter means, and a source ofpolarized light disposed on the other side of said magneto-opticallyactive means, said source of light having a wavelength at which saidmagnetooptically active means is transparent.

3. Analog to digital signal converter apparatus comprising, incombination, a plurality of photosensitive readout devices disposedadjacent each other, each of said readout devices being divided alongthe length thereof into an equal number of coextensive portions, each ofsaid devices generating a signal While a sensitive portion thereof haslight impinging thereon, corresponding coextensive portions of all ofthe photosensitive devices being selectively sensitive and insensitivein accordance with a binary code pattern whereby light impinging uponall the coextensive portions of all of said photosensitive devices in aselected zone of portions causes the generation of a signal in onlyselected ones of said devices in accordance with the location of thesensitive and insensitive portions thereof, a source of polarized lighthaving a predetermined wavelength and plane of polarization, and meansincluding magneto-optically active means and a filter for polarizedlight interposed in the order named between the source of light and theplurality of photosensitive devices for selectively passing light toselected portions of the devices in [accordance with variations in ananalog signal to thereby selectively energize said plurality ofphotosensitive readout devices in a binary code patter-n correspondingto. the value of the analog signal.

I 4. In analog to digital voltage converte apparatus, in combination, asource of plane polarized light having a predetermined plane ofpolarization, a plurality of elongated photosensitive 1eadout devicesdisposed contiguous and substantially parallel to each other inpredetermined positions with respect to the source of light, each ofsaid readout devices generating an electrical signal while light energyreaches any portion thereof, each of said readout devices being dividedalong the length thereof into an equal number of coextensivesubstantially rectangular portions, corresponding portions of each ofsaid plurality of devices being symmetrically located to form aplurality of substantially rectangular zones spaced along the length ofthe devices, each of said zones including a portion of each of thedevices, the plurality of zones extending substantially transverse tothe longitudinal axes of the devices, the portions of each zone beingselectively masked and unmasked in accordance with a binary code patternhaving a number of digits equal to the number of said readout devices,light impinging on any masked portion producing no signal in the readoutdevice, polarized light analyzer means disposed with one side thereofadjacent said plurality of photosensitive readout devices and betweenthe devices and the source of light for excluding only light having aplane of polarization at a predetermined angle to said predeterminedplane of polarization, a transparent member of magneto-optically activematerial disposed adjacent the other side of said analyzer means betweenthe analyzer means and the source Otf light, said magnetoopticallyactive member having a plurality of leads extending thereacross atspaced intervals corresponding to the width of the zones extendingacross the photosensitive readout devices, the leads being centrallydisposed with respect to the zones respectively, said active materialnormally in the absence of an external magnetic field applied theretorotating the plane of polarization of a portion ot the light firom saidsource in a manner whereby said portion is excluded by the analyzermeans and normally rotating the plane of polarization of another portionof light from said source whereby said other portion is not excluded bythe analyzer means, a current flow in any one of said leads setting up amagnetic field in the area of the magneto-optically active memberadjacent the last-named lead, said magnetic field causing a change inthe polarization of said other portion of light passing through themember area adjacent the lead whereby said last-named light is nottransmitted through the analyzer means to the zone of the photosensitivereadout devices adjacent said last-named lead.

5. Apparatus according to claim 4 including in addi tion a source ofdirect current potential, means including a voltage dividing networkoperatively connected to said source oi= direct current potential andenergized there,

from for providing a plurality of graduated voltages, adjacent ends ofsaid plurality of leads being operatively connected to different pointson said voltage dividing network whereby a plurality of diiferentpotentials are applied to the plurality of leads respectively, anamplifier having a variable input signal applied thereto and providing avariable direct current output which varies as a function of the inputsignal, and output means for the amplifier operatively connected to theother ends of all of said plurality of leads whereby a variably selectedone of said leads has substantially no current flowing therethrough as aresult of the balancing of potentials at the two ends thereof inaccordance with the instant value of said input signal, all the other ofsaid leads having current flows therethrough as a result of differencesin potentials art the ends thereotf.

6. In an analog to digital voltage converter, in combination, amplifiermeans having the analog voltage applied thereto, a source of directcurrent potential, a resistance voltage dividing network having aplurality of resistors operatively connected in series to said source ofdirect current potential, a plurality of'leads connected at adjacentends thereof to a plurality of different voltage points on said voltagedividing network, all the other ends of said plurality of leads beingconnected to the output of said amplifier means to receive a voltagetherefrom, a selected one of 'said leads having no substantial potentialdiiference existing between the ends thereof in accordance with theinstant output of the amplifier means whereby substantially no currentflows therethrough, all the remainder of said plurality of leadscarrying current flows during said instant, a thin fiat member composedof yttrium iron garnet, all of said plurality of leads being disposed atspaced intervals along the length of said member otf yttrium irongarnet, a source of plane polarized light of predetermined polarizationdisposed in predetermined position with respect to said yttrium irongarnet member whereby light from said source impinges upon said memberin areas adjacent to all of said leads, said source of plane polarizedlight having a wavelength at which said yttrium iron garnet member istransparent, light analyzer means disposed on the other side of saidgarnet member for passing only polarized light which has a plane ofpolarization corresponding to the transmission characteristics of thelight analyzer means, and photosensitive detector means disposed on theother side of said light analyzer means, flows of current in theremainder of said leads causing magnetic fields to be set up in theareas of the garnet member adjacent thereto, said magnetic fieldschanging the plane of polarization Otf light passing through the garnetmember in said areas whereby said last-named light is not transmitted bysaid analyzer means to said photosensitive detector means, polarizedlight from said source which passes through the area of said garnetmember adjacent said one lead having no magnetic field thereat havingthe plane of polarization of some out the light passing through saidlast-named area altered a predetermined amount so that some of the lightpasses through said light analyzer means to said photosensitive detectormeans.

7. In analog to digital voltage converter apparatus, in combination, aplurality of photosensitive elements, a

source of plane polarized light disposed in predetermined position withrespect to said plurality of photosensitive elements, a member ofmagneto-optically active material disposed between said source ofpolarized light and said plurality of photosensitive elements, saidsource of light having a wavelength at which said magneto-opticallyactive material is transparent, a filter element for polarized lightdisposed between said member of active material and said photosensitiveelements, said filter element excluding only polarized light having aplane of polarization at a predetermined angle to the plane ofpolarization of the light emitted from said source, and means having theanalog voltage applied thereto and operatively connected to the memberof active material for selectively creating magnetic fields in portionsof said member of active material which change the polarization of lightpassing through the member in said portions, said portions varying inposition with variations in the instant value of the analog voltage.

8. Apparatus according to claim 7 wherein said plurality ofphotosensitive elements are additionally masked in predeterminedpatterns whereby the plurality of photosensitive elements provide aplurality of output voltages in digitally coded form which varyselectively in accord ance with variations in the areas of the member ofactive material in which the magnetic fields are set up.

9. An analog to digital voltage converter comprising, in combination, aplurality of photosensitive devices each generating a signal While lightimpinges on a sensitive portion thereof, said plurality ofphotosensitive devices being disposed adjacent each other, each of saidphotosensitive devices being divided along the length thereof into anequal number of coextensive portions, the portions of each of saidphotosensitive devices being selectively insensitive and sensitive toprovide a total pattern corresponding to a binary code having a numberof digits equal to the number of said photosensitive devices, filtermeans for polarized light disposed with one side thereof adjacent saidplurality of photosensitive devices for excluding only polarized lighthaving a predetermined plane of polarization, yttrium iron garnet filmdisposed adjacent said filter means on the other side thereof, a sourceof plane polarized light disposed on the other side of said yttrium irongarnet film in predetermined position with respect thereto and having awavelength at which said garnet film is transparent, polarized lightfrom said source passing through said film in the absence of an externalmagnetic field applied thereto with predetermined changes in thepolarization thereof and with at least a portion of the light passingthrough said filter means and impinging on said photosensitive devices,a plurality of leads extending at spaced intervals and in substantiallyparallel lines across the suriace of said garnet film, each of saidleadscreating an external magnetic field in the area of the filmadjacent thereto while current flows in the lead, analog voltage inputmeans operatively connected to all of said leads, means operativelyconnected to all of said leads for applying graduated voltages theretowhereby only one of said leads has no current flow therein in accordancewith the instant value of an analog voltage, light passing through thearea of said garnet film adjacent said last-named lead and impingingupon said plurality of photosensitive devices causing signals to begenerated therein in accordance with the pattern of sensitive andinsensitive portions of said devices adjacent the area of the filmthrough which the light passes, light impinging on the other areas ofthe garnet film having the polarization thereof shifted to saidpredetermined plane whereby light passing through said other areas doesnot pass through the filter means to the photosensitive devices.

10. In an analog to digital voltage converter, in combination, meansforming a film of yttrium iron garnet, means operatively connected tosaid film for setting up a plurality of magnetic fields at spacedintervals along said film, said intervals corresponding to discretechanges in the value of the analog voltage, no magnetic field being setup in the film at a position corresponding to the instant value of theanalog voltage, a source of polarized light disposed in predeterminedposition with respect to said garnet film on one side thereof, saidsource of light having a wavelength at which said garnet film istransparent, filter means for polarized light disposed adjacent saidgarnet film on the other side thereof, and a plurality of photosensitivedevices disposed on the other side of said filter means, the magneticfields in the areas of the garnet film causing light passing through theareas of the garnet film in which the magnetic fields exist to beshifted in polarization and not to pass through the filter means to thephotosensitive devices, light from said source which passes through thearea of the garnet film having no external magnetic field createdtherein passing through the filter means to the photosensitive devices.

11. Apparatus according to claim 2 wherein said magneto-optically activemeans is composed of yttrium iron garnet.

12. Apparatus according to claim 3 wherein said magnetic-opticallyactive means is composed of yttrium iron garnet.

13. Apparatus according to claim 4 wherein said magneto-optically activemember is composed of yttrium iron garnet.

14. Apparatus according to claim 2 wherein said magneto-optically activemeans is composed of gadolinium iron garnet.

15. Apparatus according to claim 3 wherein said magneto-optically activemeans is composed of gadolinium iron garnet.

16. Apparatus according to claim 4 wherein said magneto-optically activemember is composed of gadolinium iron garnet.

17. Apparatus according to claim 4 wherein said magneto-optically activemember is additionally characterized as containing discrete magneticdomains normally characterized by magnetic moments of both polaritieswith re spect to a selected direction of measurement, a magnetic momentof one polarity rotating the electric vector of the light in one angulardirection and a magnetic moment of the opposite polarity rotating saidvector of the light in the opposite angular direction, the applicationof an exernal magnetic field causing substantially all the magneticmoments to assume the same polarity whereby all the light has the vectorthereof rotated in the same angular direction to provide a plane ofpolarization at said predetermined angle.

18. Analog-to-digital signal converter apparatus comprising a source ofpolarized light, a plurality of elongated photosensitive readoutelements disposed adjacent each other, masking means for excluding lightfrom portions of all of said readout elements in accordance with abinary code pattern thereby providing a pattern of areas that areresponsive and non-responsive, respectively, to light in accordance witha binary code, an analyzer disposed between said light source and saidphotosensitive elements, said analyzer being oriented at an angle ofextinction with respect to the plane of polarization of said source ofpolarized light, and magneto-optical means disposed between said sourceof polarized light and said analyzer means for selectively rotating thepolarization of a portion of said light in accordance with the value ofthe analog signal to be digitized to selectively enable light to passthrough selected portions of said analyzer.

l9. Analog-to-digital signal converter apparatus comprising a source ofpolarized light, a plurality of elongated photosensitive readoutelements disposed adjacent each other, masking means for excluding lightfrom portions of all of said readout elements in accordance with abinary code pattern thereby providing a pattern of areas that areresponsive and non-responsive, respectively, to light in ac- 11 cordancewith the binary code, an analyzer disposed between said source ofpolarized light and said photosensitive elements, said analyzer beingoriented at an angle of extinction with respect to the polarization oflight from said light source, and magneto-optical means disposed betweensaid light source and said analyzer means, said magneto-optical meanscomprising a magneto-optically active film and a plurality of lead meansdisposed adjacent thereto, means connected to all of said plurality oflead means for causing current flow in all but a selected one of saidlead means selectively in accordance with the value of the analog signalto be digitized, those of said lead'means having current flowing thereincreating magnetic fields in the adjacent regions of saidmagneto-optition ofa selected portion of the light from said source toselectively enable light to pass through selected portions of saidanalyzer.

References Cited in the file of this patent UNITED STATES PATENTS2,834,005 Ketchledge May 6, 1958 2,855,539 Hoover Oct. 7, 1958 2,968,799Smith Ian. 17, 1961 OTHER REFERENCES Magnetic Domains by LongitudinalKerr Effect, by Chas. A Fowler, Jr., and Edward M. Fryer, PhysicalRecally active film to selectively rotate the plane of polariza- 15view, April 1, 1954, pages 5256.

1. IN ANALOG TO DIGITAL SIGNAL CONVERTER APPARATUS, IN COMBINATION, ASOURCE OF LIGHT HAVING A PREDETERMINED PLANE OF POLARIZATION, APLURALITY OF LIGHT SENSITIVE ELEMENTS DISPOSED IN PREDETERMINEDPOSITIONS WITH RESPECT TO EACH OTHER AND WITH RESPECT TO SAID SOURCE OFLIGHT, EACH OF SAID LIGHT SENSITIVE ELEMENTS HAVING SENSITIVE ANDINSENSITIVE PORTIONS AND GENERATING AN ELECTRICAL OUTPUT SIGNAL WHILELIGHT ENERGY IMPINGES ONLY ON A SENSITIVE PORTION THEREOF, THE SENSITIVEAND INSENSITIVE PORTIONS OF ALL SAID PLURALITY OF LIGHT SENSITIVEELEMENTS BEING ARRANGED IN A PREDETERMINED PATTERN WHEREBY LIGHTREACHING CERTAIN SELECTED SENSITIVE PORTIONS PROVIDES OUTPUT SIGNALSFROM THE LIGHT SENSITIVE ELEMENTS WHICH ARE DIGITALLY CODED TOCORRESPOND TO A QUANTITY IN ACCORDANCE WITH THE ARRANGEMENT OF THESELECTED SENSITIVE PORTIONS WITH RESPECT TO EACH OTHER AND LIGHTREACHING CERTAIN OTHER SELECTED SENSITIVE PORTIONS PROVIDES OTHER OUTPUTSIGNALS FROM THE LIGHT SENSITIVE ELEMENTS WHICH ARE DIGITALLY CODED TOCORRESPOND TO ANOTHER QUANTITY IN ACCORDANCE WITH THE ARRANGEMENT OF THEOTHER SELECTED SENSITIVE PORTIONS WITH RESPECT TO EACH OTHER, LIGHTFILTERING MEANS INTERPOSED BETWEEN THE SOURCE OF LIGHT AND THE PLURALITYOF LIGHT SENSITIVE ELEMENTS AND EXCLUDING ONLY LIGHT HAVING A PLANE OFPOLARIZATION AT A PREDETERMINED ANGLE WITH RESPECT TO SAID PREDETERMINEDPLANE OF POLARIZATION, AND ANALOG SIGNAL MEANS INCLUDING MAGNETOOPTICALLY ACTIVE MEANS INTERPOSED BETWEEN THE SOURCE OF LIGHT AND SAIDLIGHT FILTERING MEANS FOR SELECTIVELY CONTROLLING THE PLANE OFPOLARIZATION OF LIGHT REACHING THE LIGHT FILTERING MEANS IN SELECTEDAREAS THEREOF SELECTED IN ACCORDANCE WITH THE VALUE OF THE ANALOG SIGNALTO THEREBY VARIABLY SELECT GROUPS OF SENSITIVE PORTIONS TO BEILLUMINATED IN ACCORDANCE WITH VARIATIONS IN THE VALUE OF THE ANALOGSIGNAL.